It necessary for many semi-conductor devices to be packaged in a hermetically sealed environment, and so any connections into the package need to pass through a hermetically sealed feed-through. Optical fibres may be used where it is necessary to conduct light to or from a package, in which case the fibre needs to be close to the device and has to pass through the package wall. The fibre itself needs to be sealed to the feed-through because otherwise there is a leakage path along the fibre between the fibre and the usual protecting sheath. Sealing the fibre to the feed-through presents problems; firstly the fibre is delicate and can be damaged by the handling processes of stripping, metalising and soldering that are typical in the preparation of feed-through, and then, once the protecting sheath is removed the fibre may be subject to stress unless the sheath is also secured to the feed-through to maintain continuity of strength and covering for the fibre.
A known method of overcoming these problems is to incorporate a transmissive window with the package device arranged to transmit or receive light through it but this has been found to cause difficulty in aligning the internal and external devices.